Unstash in loop only if identifier is available

In case unstashAll is called with a Set containing a null value we get
an ugly message in the log:

Unstash failed: null (Could not instantiate {name=null} for UnstashStep(name: String):

This can be avoided by unstashing only in case we have an ID.

src/com/sap/piper/Utils.groovy

@@ -54,6 +54,7 @@ def stashWithMessage(name, msg, include = '**/*.*', exclude = '') {
 }
 
 def unstash(name, msg = "Unstash failed:") {
+
     def unstashedContent = []
     try {
         echo "Unstash content: ${name}"
@@ -69,7 +70,9 @@ def unstashAll(stashContent) {
     def unstashedContent = []
     if (stashContent) {
         for (i = 0; i < stashContent.size(); i++) {
-            unstashedContent += unstash(stashContent[i])
+            if(stashContent[i]) {
+                unstashedContent += unstash(stashContent[i])
+            }
         }
     }
     return unstashedContent

test/groovy/com/sap/piper/UtilsTest.groovy

@@ -38,6 +38,7 @@ class UtilsTest extends BasePiperTest {
     void setup() {
 
         parameters = [:]
+
     }
 
     @Test
@@ -70,4 +71,11 @@ class UtilsTest extends BasePiperTest {
         // generated with "echo -n 'ContinuousDelivery' | sha1sum | sed 's/  -//'"
         assertThat(result, is('0dad6c33b6246702132454f604dee80740f399ad'))
     }
+
+    @Test
+    void testUnstashAllSkipNull() {
+
+        def stashResult = utils.unstashAll(['a', null, 'b'])
+        assert stashResult == ['a', 'b']
+    }
 }